This invention relates to support bearings for elongated members subjected to axial reciprocation and is more particularly directed to the provision and maintenance of an effective hydrodynamic lubricant film between the bearing and the reciprocating member.
In mechanisms wherein one or more cylindrical bearings are utilized to concentrically support an elongated member for axial reciprocation, it is essential that the interior geometry of the bearings facilitate the initial formation and subsequent continuance of a lubricant film therein of sufficient thickness to withstand the loading imparted thereto throughout the entire reciprocation of the supported member. Such requirement is particularly important in recoil systems for large caliber guns wherein the supported member is a gun tube subjected to extremely high acceleration forces during the initial few milliseconds of recoil. Since the center of gravity of the recoiling mass is often offset from the bore axis of the gun tube, the extremely rapid rise in the acceleration thereof generates a couple which pivots the gun tube into loading contact with the support bearing. During this critical loading period, the flow of the lubricant fluid into each bearing lags well behind the actual velocity of the gun tube so that the quantity thereof drawn into the bearing will, at best, form a "boundary" film, i.e., one with a thickness of one or two molecular dimensions. Experience has shown that a film of such marginal nature will not withstand the peak loading imparted thereto thereby leading to undesirable metal-to-metal contact between the bearing and the gun tube. As a result, the bearings heretofore employed to support large caliber gun tubes are unusually vulnerable to rapid and excessive wear. In many instances, such metal-to-metal bearing contact has actually prevented the gun tube from returning to the proper battery position thereof.
Experience with thrust bearings has demonstrated that certain surface profiles thereon are conductive to the formation and continuance of lubricant films of significantly increased thickness. One such profile includes a plurality of sectorshaped recesses separated by lands provided with a stepped surface, known in the art as a "Rayleigh step," along the leading edge thereof. Another configuration intended to improve the load carrying capacity of a thrust bearing surface consists of a plurality of spiral or herringbone oil-carrying grooves. However, while these surface profiles have met with some success in thrust bearings designed to support a shaft subjected to a steady state type of reciprocal movement, such is not the case in connection with recoil bearings for concentrically supporting a gun tube to which a constantly changing acceleration is applied. Furthermore, it has been found that the incorporation of a "Rayleigh step" in a cylindrical bearing surrounding a sliding shaft acerbates the "side leakage" of the lubricating fluid, i.e., the tendency to flow in a circumferential direction away from the area of maximum loading to the diametrically opposed area of minimum loading. Obviously, such leakage cannot be tolerated especially during the peak loading encountered within the initial portion of recoil.